There has been known a power feed device including: a power generator generating power that should be fed; a power feed element formed of a coil to which power generated by the power generator is fed; a resonance element coupled to the power feed element by electromagnetic induction; and a variable matching unit including a function for impedance matching at a point of feed of the power to the power feed element, in which the power feed element is formed so that a diameter thereof can be changed, and the variable matching unit can change the diameter of the power feed element (see Patent Document 1, for example).
Further, there has been known a power feeding apparatus including: a power generator generating power that should be fed; a power transmission element to which power generated by the power generator is fed; an impedance detector detecting an impedance on the power feeding side; a variable matching unit including a function for impedance matching at a point of feed of the power to the power transmission element; a storage unit storing impedance characteristic estimation information for estimating an impedance characteristic on the power reception side from an impedance characteristic on the power transmission side therein as a reference table; and a control unit obtaining a state of the variable matching unit that should be adjusted from at least information on detected impedance and information on the reference table in the storage unit and outputting a control signal to the variable matching unit so as to be in the obtained state (see Patent Document 2, for example).
Further, there has been known a non-contact power transmission apparatus including: an AC power supply; a primary coil connected to the AC power supply; a primary side resonant coil; a secondary side resonant coil; a secondary coil; a load connected to the secondary coil; and an impedance variable circuit provided between the AC power supply and the primary coil, in which the primary coil, the primary side resonant coil, the secondary side resonant coil, the secondary coil, and the load constitute a resonant system and include a state detecting means to detect a state of the resonant system, and in the impedance variable circuit, an impedance is adjusted so that an input impedance in a resonant frequency of the resonant system and an impedance on the AC power supply side rather than the primary coil may be matched based on a detection result of the state detecting means (see Patent Document 3, for example).
Further, there has been known a radio power transfer device being a radio power transfer device to transmit power by generating magnetic field resonance between a power transmission unit including a power transmission coil and a power reception unit including a power reception coil, the radio power transfer device including: a changing means that changes transfer efficiency of power by changing a predetermined power transmission/reception element provided in either the power transmission unit or the power reception unit; a transfer efficiency calculating means that calculates transfer efficiency before and after the change by the changing means; and a judging means that judges whether or not the transfer efficiency is improved after the change by comparing the transfer efficiency before the change and the transfer efficiency after the change that are calculated by the transfer efficiency calculating means, in which the judgment by the judging means and the change by the changing means are performed repeatedly, to thereby improve the transfer efficiency (see Patent Document 4, for example).